V. M. Pashkov

Search for and study of phase transitions in some representatives of the APb2 X 5 family

Single crystals of KPb2Cl5, RbPb2Cl5, and RbPb2Br5 are grown and studied using optical polarization methods. The heat capacity of the crystals is investigated by differential scanning microcalorimetry, and the birefringence and the angle of rotation of the optical indicatrix are measured. The measurements are performed in the temperature range 270–640 K. It is found that KPb2Cl5 undergoes a first-order ferroelastic phase transition at T0↑ = 530 K, T0↓ = 528 K, and ΔH = 1000 ± 200 J/mol. The transition is accompanied by twinning and a change in symmetry mmm ⇄ P21/c. The RbPb2Cl5 crystal remains monoclinic up to the melting temperature. The RbPb2Br5 compound belongs to the I4/mcm tetragonal modification and does not undergo structural transformations.

Investigation of the influence of gradual substitution K ↔ Rb on the structure and phase transition in KxRb1 − xPb2Br5 solid solutions

Single crystals of KxRb1 − xPb2Br5 solid solutions have been grown and investigated using X-ray diffraction and polarized light microscopy in the temperature range 270–640 K. The regions of existence of the tetragonal (I4/mcm) and monoclinic (P21/c) phases have been determined. It has been demonstrated that the partial introduction of rubidium into KPb2Br5 leads to an increase in the temperature of the ferroelastic phase transition (P21/c↔mmm), so that it approaches the melting temperature. In the solid solutions with x ≈ 0.4−0.5, the temperature region of existence of the orthorhombic phase is narrowed to ≈ 1−2 K. It has been revealed that an increase in the potassium content in the material results in a considerable increase in the incorporation coefficient of Er3+ ions.

Vibrational spectrum and elastic properties of KPb2Cl5 crystals

The Raman spectra and elastic moduli of KPb2Cl5 crystals were studied experimentally. The results are interpreted using a parameter-free model of the crystal lattice dynamics with inclusion of the multipole moments of the electron shells of ions. The calculated and experimental results are in good agreement. It is shown that not only the halogen ions but also the heavy cations make a significant contribution to the eigenvectors of high-frequency lattice vibration modes, which accounts for the relatively low frequencies of these modes.

Phase transition in a KPb2Br5 crystal

Crystals of the KPb2Br5compound are investigated using polarized light microscopy and calorimetry. The birefringence and the angle of rotation of the optical indicatrix are measured in the temperature range 270–620 K. It is found that the KPb2Br5 crystal undergoes a first-order ferroelastic phase transition at temperatures T0↑ = 519.5 K and T0↓ = 518.5 K with a change in the enthalpy ΔH = 1300 ± 200 J/mol. This transition is accompanied by both twinning and the symmetry change mmm ↔ P21/c. It is revealed that the angle of rotation of the optical indicatrix exhibits an unusual behavior under variations in the temperature due to a strong temperature dependence of the birefringence.